If you're using your own letterhead paper for your documents (invoices, quotes, etc.), you may need to adapt the margins to make sure the text doesn't mess with your layout.
To adapt the margins, you have to go to Settings > Document templates and scroll to the "Define the margins for your documents" panel.
You then simply have to customize your margins and click on Save. You can also tick an option to define a specific top margin for the first page of your documents (this can be useful if you need a bigger margin on the first page because you have a multi-pages letterhead paper with your logo and address on the first page).
The next documents you will create will show the customized margins.